1. Field of the Invention
The present invention relates to a thermoelectric material and a thermoelectric device.
2. Description of the Related Art
In recent years, from increasing concerns to global environmental problems, thermoelectric cooling devices making use of the Peltier effect are highly interested as CFC-free coolers. In addition, in order to reduce carbon dioxide emission, thermoelectric generating devices making use of the Seebeck effect that can provide power generators using unused waste heat energy are highly interested.
Conventionally, typical thermoelectric materials used in room temperature include Bi—Te-based single crystal and polycrystal materials for their high efficiency. When a thermoelectric device is fabricated using both of p-type and n-type Bi—Te-based thermoelectric materials, Bi—Te added with Se is generally used as the n-type Bi—Te. On the other hand, typical thermoelectric materials used at temperatures higher than room temperature include Pb—Te-based materials for their high efficiency. Elements such as Se (selenium), Pb (lead) and Te (tellurium) used in the Bi—Te-base or Pb—Te-based thermoelectric devices are toxic and harmful to human bodies and further undesirable to the global environmental problems. Accordingly, harmless thermoelectric materials have been studied in place of the Bi—Te-based and Pb—Te-based thermoelectric materials.
A figure of merit Z of a thermoelectric material is represented by the following equation (1):Z=α2/(ρκ)  (1)where, α denotes a Seebeck coefficient of the thermoelectric material, ρ denotes an electrical resistivity of the thermoelectric material and κ denotes a thermal conductivity of the thermoelectric material. The figure of merit Z has a dimension of an inverse of temperature, and thus the product of Z and an absolute temperature T has a dimensionless value. The ZT value is referred to as a dimensionless figure of merit. A thermoelectric material having a higher ZT value has higher thermoelectric conversion efficiency. As will be seen from the equation (1), the thermoelectric material is required to have higher Seebeck coefficient, lower electrical resistivity and lower thermal conductivity.
As one of thermoelectric materials in which the toxic materials such as described above are not contained or reduced as far as possible, half-Heusler compounds having a MgAgAs crystal phase are known (see, for example, Proc. of 18th International Conference on Thermoelectrics (1999), p. 344). Conventional half-Heusler compounds have not exhibited sufficient ZT values, however, and therefore an improvement in the ZT value is demanded.